1. Field of the Art
This invention is intended to provide a process for producing a highly water absorptive polymer having excellent water absorbing properties with extreme ease and at low cost.
The polymer obtained according to the production process of the present invention can be swelled by absorption of a large amount of water and is insoluble in water, and has a great strength of the swelled gel, and therefore it can be used advantageously for production of various absorbing materials and various materials which are to be used under swelled state with water being absorbed.
2. Prior Art
Materials such as paper, pulp, nonwoven fabric, spongy urethane resins and the like have hitherto been used as water retentive materials for a variety of sanitary goods such as a sanitary napkin, paper diaper and the like and a variety of agricultural materials. However, these materials have a water absorption capacity of no more than 10-50 times their own weight, which causes problems that an extensively increased bulk of the material is required for absorbing or retaining a large amount of water and that water is easily released from the material in which water has been absorbed on pressing it.
There have recently been proposed a variety of highly water absorptive polymer materials and production methods thereof in order to settle the aforementioned problems of the water absorptive materials of this kind. For instance, there have been proposed a graft polymer of starch (Japanese Patent Publication No. 46199/1978, etc.), a denaturated cellulose (Unexamined Published Japanese Patent Application No. 80376/1975, etc.), a crosslinked water soluble polymer (Japanese Patent Publication No. 23462/1968, etc.), a self-crosslinking polymer of an alkali metal salt of acrylic acid (Japanese Patent Publication No. 30710/1979, etc.), crosslinked type polyacrylic acid alkali metal salt (Unexamined Published Japanese Patent Publication No. 71909/1983, Japanese Patent Publication No. 17328/1985).
However, some of these highly water absorptive polymer materials have a number of problems in practical use or production on an industrial scale such that they are still insufficient in amount of water to be absorbed, with the gel strength after water absorption being small, and that the polymer obtained by drying is extremely hard and cannot be easily crushed thereby requiring a great mechanical crushing force. Also, some of those mentioned above employ a large amount of hydrocarbon type solvent, thus involving a serious problem on safety.
A process wherein no hydrocarbon solvents are used may involve polymerizing an aqueous solution of an alkali or ammonium salt of acrylic acid in the absence of a crosslinking agent as disclosed in Unexamined Published Japanese Patent Publication Nos. 58208/1980 and 160302/1983. According to this process, however, it is difficult to obtain a polymer having remarkable water absorbing properties since no crosslinking agent is used and crosslinking takes place due to the self-crosslinking of the acrylate or to obtain a polymer of stable quality from the viewpoint of process operation.
Unexamined Published Japanese Patent Publication Nos. 84304/1980, 108407/1980, 133413/1980, 84632/1981 and 91837/1981 disclose a process for producing highly water absorptive acrylate polymers wherein 50 mol % or more of the carboxyl group has been neutralized with an alkali metal salt in the presence of a polyhydric alcohol and/or a water-soluble and/or water-dispersible surfactant. In accordance with this process, however, the degree of neutralization of the carboxyl group is as high as 50% or more (ordinarily 75% or more) so that, when a sodium salt, for example, is used as the alkali metal salt, the monomer concentration in the aqueous solution can be increased at most up to about 45% by weight and thus enormous energy is required to evaporate a large amount of water contained in the polymer obtained. Furthermore, this process is not so attractive from the process viewpoint since it requires a time as long as 3 to 8 hours for completing polymerization (under such polymerization conditions as in the present invention).
Unexamined Published Japanese Patent Publication No. 71907/1983, on the other hand, discloses a process wherein the monomer concentration can be increased to 55 to 80% by weight with the use of a potassium salt of acrylic acid and the water contained in the product polymer is vaporized by utilizing heat of reaction during polymerization. Nevertheless, this process only affords a polymer having a water absorption capacity not higher than 50 to 60 times its own weight and further tends to involve high cost due to the use of a potassium salt as an alkali metal salt